1. The Field of the Invention
The present invention relates to the catalytic elimination of organic waste and pollutants found in the exhaust of diesel engines. More particularly, the present invention relates to methods and systems for safely operating a diesel engine within a methane-rich environment, such as a mine.
2. The Relevant Technology
Internal combustion engines are known to emit pollutants that are toxic to humans. Extensive exposure to pollutants can cause health related problems in humans and are a significant source of air pollution in developing and developed countries, such as the United States. Diesel engines are particularly common source of pollution. The black smoke, or soot, is the most visible emission, but other, less visible pollutants are also present. The emissions from a diesel engine are composed of three phases: solids, liquids, and gases. The combined solids and liquids are called particulates, or total particulate matter (“TPM”), and are composed of dry carbon (soot), inorganic oxides (primarily as sulfates), and liquids.
When diesel fuel is burned, a portion of the sulfur is oxidized to sulphate which, upon reaction with the moisture in the exhaust, becomes H2SO4. The liquids are a combination of unburned diesel fuel and lubricating oil called the soluble organic fractions (SOF) or volatile organic fractions (VOF) which form discrete aerosols and/or are adsorbed within the dry carbon particles. Gaseous hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur dioxide are the constituents of a third phase. The emissions from burned diesel fuel can also vary on the amount of lubricant in the diesel fuel. Other engines may generate a dry exhaust in which the SOF is lower, with the balance being primarily dry carbon. Diesel emission are clearly more complex than those of gasoline engines and, hence, their catalytic treatment is more complicated and requires new technology.
In response to pollution caused by gasoline-powered internal combustion engines, catalytic converters have been developed to reduce the levels of pollutants that are emitted into the environment from burning gasoline. Catalytic converters are typically positioned in-line with the exhaust system of the gas engine and are generally able to catalytically convert most of the unburned hydrocarbons into CO2 and water.
Conventional catalytic converters contain palladium or platinum, which are coated on top of carrier beads or pellets made of inert and heat-resistant materials in order to increase the surface area of the active catalyst and keep the particles from escaping through the exhaust pipe. Coating the catalytic metal on a substrate also decreases the cost of the catalyst particles since most catalytic metals are much more expensive than a substrate. Because lead-based additives can “poison” or destroy the usefulness of the catalyst, such additives have been banned.
Although modern catalytic converters can be used to convert unburned hydrocarbons to carbon dioxide (CO2) and water, they are generally only feasible for use in gasoline-powered vehicles. Existing catalytic converters are less suitable for use with diesel engines. The type of fuel and the manner in which it is burned in a diesel engine produce substantial quantities of soot and other unburned hydrocarbons, which are too plentiful to be efficiently converted into CO2 and water using existing catalytic converters. Although diesel engines are known to significantly pollute, diesel engines have been largely exempted from the stringent air quality guidelines for economic reasons. One reason is that diesel engines are used for long-haul shipping, such as such as by tractor-trailers and trains. Their elimination might cause dire economic problems.
In the coal mining industry, however, the use of diesel engines is heavily regulated. Because of the natural gas present in mines and the pollutants created by diesel engines, enclosed areas such as mines create a particularly sensitive environment for operating diesel engines. As a result, the U.S. Department of Labor Mine Safety and Health Administration (MSHA) has promulgated federal regulations governing the use of diesel engines in underground mines. Federal regulations found in 30 C.F.R. Parts 7, 36, 70, and 75 provide safety standards for underground coal mines. The Regulations stipulate various conditions including the fuel-air mixture ratios, operating temperatures, and levels of pollutants in the exhaust. MSHA also requires special equipment such as flame arrestors on the intake and exhaust.
MSHA has disseminated stringent rules regarding the emission of various pollutants. For instance, recent MSHA standards require that diluted exhaust gases from diesel engines contain no more than 0.5% by volume of carbon dioxide; 0.01% by volume of carbon monoxide; 0.0025% by volume of oxides of nitrogen (calculated as equivalent nitrogen dioxide); or 0.0010 percent, by volume, of aldehydes (calculated as equivalent formaldehyde) under any condition of engine operation prescribed by MSHA. Proper testing of a diesel engine also requires measuring the levels of methane in the exhaust gas. Unlike normal operating conditions, operating a diesel engine in a methane-rich environment increases the chances that the exhaust will contain methane.
Preventing explosions and reducing pollution in underground mines is another challenge when operating a diesel engine. The coal found in mines can produce high levels of methane. The methane becomes trapped in the enclosed mine, thus creating a hazard for humans and machines. For example, mines have been known to contain air that is 8% by volume methane, and in some cases, the concentrations can reach 12% by volume of methane. Once released by the coal, the methane typically remains in the mine because of poor ventilation.
Because methane is combustible, diesel engines operating in or around mines can cause pockets of methane to explode. In addition, diesel engines use the surrounding air for air intake to complete combustion of the diesel fuel. Methane in the surrounding air is sucked into the diesel engine where it can explode. Because methane is more explosive than diesel fuel, the methane presents a risk if found in high concentrations.
Methane creates a particular risk of explosion due to incomplete seating of valves and fuel blow-by caused thereby. All diesel engines have three phases of emissions. In the first phase, incomplete pre-ignition causes soot to be jammed in the valve seats, which prevents complete seating of the valves. This allows pressurized air-fuel mixture to pass through the valve during each stroke, resulting in the blow-by of approximately 20% fuel and lubricating oil. In addition, soot that is formed due to incomplete mixing of the fuel-air mixture produces cold spots in the piston and wall crevices. Parts of the soot become fused to the valve seat, becoming elemental carbon, and turned white-hot due to friction. Elemental carbon is formed during the second and third phases of the emission cycle. When a diesel engine operating under normal conditions is utilized in a mine high in methane gas, the methane gas, in addition to the fuel-air mixture, forms part of the blow-by that escapes through the unseated valve. Miliseconds after the top of compression, the methane can interact with the white-hot elemental carbon fused to the valve seat, at such high temperatures, can yield a great explosion. Sparks comprising red hot carbon from the valves can literally fly out of the emissions, potentially causing fires or explosions outside of the diesel engine. For this reason spark arrestors are commonly used.
Mines create a particularly problematic environment for operating a diesel engine because the engine's air intake draws in methane from the surrounding air. Unmixed, the increase in fuel-air ratios increases the emission of toxic gases such as carbon monoxide and nitrogen dioxide. Further compounding the problem is the fact that coal mines are often enclosed and poorly ventilated.
While techniques exist for properly operating a diesel engine in or near a mine, these techniques are often costly and undesirable. For example, the engine must be operated at lower than full power and/or be modified to include expensive and complicated equipment.
Therefore, what is needed is a cost effective system that can prevent explosions in or near diesel engines running in a methane-rich environment and that can reduce the amounts of non-combusted fuels and pollutants in the exhaust gases.